Technical Field
The present disclosure relates to an organic light emitting diode (hereinafter, referred to as “OLED”) display device.
Description of the Related Art
Each of a plurality of pixels constituting an OLED display device includes an OLED having an organic light emitting layer between an anode and a cathode and a pixel circuit that independently drives the OLED. The pixel circuit includes a switching thin film transistor (hereinafter, referred to as “TFT”), a capacitor, and a driving TFT. The switching TFT charges the capacitor with a data voltage in response to a scan pulse. The driving TFT regulates emission of the OLED by controlling the amount of current supplied to the OLED according to the data voltage charged in the capacitor.
Such an OLED display device is comprised of an X*Y matrix including x number of pixel rows and y number of column unit pixels on a screen. That is, each horizontal pixel row is comprised of y number of pixels and each vertical pixel row is comprised of x number of pixels. The OLED display device displays an image in a single frame by writing data in order from a first pixel row to a lowermost xth row unit pixel on a screen.
Meanwhile, in the organic light emitting layer constituting the OLED, a hole injection layer and a hole transporting layer adjacent to the anode is configured as a common single layer in all of the pixels constituting the OLED display device. However, while the OLED display device writes data to the first pixel row through the lowermost pixel row in order, there is a time when a voltage difference is generated between anodes of adjacent pixels. Due to a voltage difference in anode between a pixel including a high-potential anode and a pixel including a low-potential anode, an unintended leakage current flows toward the pixel including a low-potential anode through the common single layer. The leakage current may cause a set value of a data voltage applied to an Nth horizontal pixel row to be deviated from the manufacturer's intention. Such a data voltage deviation caused by the leakage current becomes a big problem when a resistance of the common single layer decreases.
Meanwhile, in the OLED display device, a problem occurs as pixels may have different driving TFT threshold voltages Vth and mobility due to process variation. Further, a voltage drop of a high-potential voltage VDD occurs, causing an amount of current driving the OLED to be changed. Thus, a luminance deviation is generated between pixels. Generally, an initial driving TFT characteristic deviation generates stain or patterns on a screen and a driving TFT characteristic deviation due to deterioration that occurs over time when driving the OLED reduces the lifespan of an OLED display panel or generates a residual image. Accordingly, there have been continued attempts to reduce a luminance deviation between pixels and thus improve an image quality by introducing a compensation circuit that compensates a driving TFT characteristic deviation and a drop voltage of a high-potential voltage VDD.